1. Field of the Invention
The present invention relates to systems and methods for increasing compression of data streams containing image data, and in particular to systems and methods that increase compression by predicting values for certain low frequency coefficients of pixel block transforms.
2. Description of the Related Art
Many important image compression methods process images as independent blocks of pixels. For example, such families of compression standards as JPEG, MPEG, H.320, and so forth, specify a step involving discrete cosine transformation ("DCT") of independent, non-overlapping 8.times.8 blocks of pixels in the source image followed by quantization of the resulting transform coefficients. See, e.g., Jack, 1996, Video Demystified, HighText Interactive Inc., San Diego, Calif. The quantized transform coefficients are transmitted from a transmitter-encoder to a receiver-decoder.
Such transformation and quantization together achieve compression by exploiting the significant regularities and correlations that typically occur between the values of pixels in 8.times.8 blocks. However, such methods ignore any regularities and correlations that may occur between pixels in different pixel blocks which are treated as independent by these methods.
Certain work has been reported which attempts to recognize image regularities at scales of a pixel block or larger. Exemplary of these are Pennebaker et al., 1993, JPEG Still Image Compression, Van Nostrand Reinhard, chap. 16, which discloses fitting quadratic surfaces to the average values of pixels (equivalent to the "DC", or lowest order, transform coefficient) in adjacent blocks, a computationally complex process; Lakhani, 1996, "Improved Image Reproduction from DC Components", Opt. Eng. 35:3449-2452, which discloses equations for predicting low frequency transform coefficients from DC coefficients that are improved from those in the JPEG standard; and Jeon et al., 1995, Blocking Artifacts Reduction in Image Coding Based on Minimum Block Boundary Discontinuity, Proc SPIE 2501:189-209, which discloses a complex and computationally expensive iterative method for interpolating pixels in order to zero block boundary discontinuities.
This reported work suffers from one or more problems, such as not being directed to maximally improving image compression, ignoring or at best inadequately treating regularities that may exist at scales in an image greater than a pixel block, requiring excessive computational resources, and so forth. What is needed, therefore, is a computationally efficient method and system directed primarily to achieving increased data compression by exploiting additional regularities and correlations in images not exploited by known compression methods and standards.
Citation of a reference herein, or throughout this specification, is not to construed as an admission that such reference is prior art to the Applicant's invention of the invention subsequently claimed.